The availability of electrical devices during fires can have lifesaving implications. Exit signs and emergency lights guide where to go in an emergency. Hard-wired fire alarms alert people to an emergency situation. In hospitals and nursing homes, electricity is needed to power devices that are directly in use to sustain life. For a petroleum or chemical plant, operations of emergency electrically operated shutoff valves in a fire are critical to allowing safe shutdown of a plant before a fire can have catastrophic effects.
Presently, most electrical wires are at risk in a fire-related emergency. Most wiring is not designed to sustain operation at high temperatures experienced in a fire. Even more wiring is ill prepared to sustain operation in a fast-rising temperature environment, such as environments where the temperature increases by hundreds of degrees each minute. During a fire-related emergency, this wiring is prone to failure.
Some wiring is designed to survive for up to two hours when exposed to a flame at up to 1850 degrees Fahrenheit. This wire is commercially available for NFPA 70 (Natiional Electrical Code) applications such as Article 695 for Fire Pumps and Article 700 for emergency systems. The temperature profile used for these applications is per ASTM E 119, which slowly raises the temperature to 1000° F. at 5 minutes to 1700° F. at 1 hour and 1850° F. at 2 hours. A test method to monitor cable circuit integrity with the ASTM E 119 temperature profile is in Underwriters Laboratories (UL) 2196. However, this same wiring will typically fail within ten minutes in a fast-rising temperature scenario, even if that temperature never rises above 2000 degrees Fahrenheit. Part of the reason for this disparity is that these cables can have a copper sheath or armor, that will melt, as well as a copper conductor. Another reason for the disparity is that a fast-rising temperature environment exposes wiring to significant thermal heat flux, sometimes exceeding 50,000 BTU/sq.ft.-hr. Most wiring is not designed to survive a fast-rising temperature environment.
For a chemical or petrochemical application, many flammable liquids may be present. These flammable liquids are the reason for the fast rise temperature profile, which simulates a hydrocarbon pool fire. The temperature profile is from UL 1709 and ASTM E 1529 that specifies a rapid rise temperature of ambient temperature to 2000° F. within 5 minutes, and holding at 2000° F. for the duration of the test. American Petroleum Institute publication 2218 “Fireproofing Practices in Petroleum and Petrochemical Processing Plants” section 6.1.8.1 states that electrical, instrumentation, and control systems used to activate equipment needed to control a fire or mitigate its consequences (such as emergency shut down systems) should be protected from fire damage for 15 to 30 minutes of fire exposure functionally equivalent to the conditions of UL 1709. The procedure in UL 1709/ASTM E 1529 specifies a totally enclosed chamber with a specified heat flux of 65,000 BTU/sq.ft.-hr and 50,000 BTU/sq.ft.-hr respectively. Since the test method in UL 1709/ASTM E 1529 is for structural steel, the circuit integrity method in UL 2196 is used to monitor cable operability.
There are other test methods that can simulate the temperature profile of UL 1709, but are not enclosed. One such method is IEC 60331-11 (formerly 331) which has an open flame. The flame temperature can be 2000° F., but because of convection, radiation, and conductance, one point on the test sample may be 2000° F., but the other side can be many hundreds of degrees lower. Another test method is MIL-DTL-25038 (formerly MIL-W-25038), which has a shake and bake test at 2000° F., which is similarly not enclosed. Cables that may pass these test tests will typically fail within 10 minutes in the UL 1709 test method.
One type of wiring that is designed to survive a fast-rising temperature is stainless steel mineral insulated (MI) cable with nickel conductor. MI cable, as the name implies, has compacted minerals located between a solid conductor and a solid metal tube outer layer. The solid conductor, as well as the mineral insulation, and metal tube make MI cable difficult to handle. Also, due to the mineral insulation, very special tools are required to terminate the MI cable connection. This MI cable is not available in long lengths, and has a very high electrical resistance due to the nickel conductor. This increased resistance requires an increase in conductor size, which limits lengths further, and makes the MI cable costlier and even harder to handle. The solid conductor is susceptible to breakage due to fatigue of the metal when it is repeatedly bent as is required for value maintenance. Finally, MI cable is susceptible to failure during exposure to moisture or water and any susceptibility to failure is undesirable in emergency power cables.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.